1. Field of the Invention
The present invention relates to an optical scanner and a polygon mirror used in this optical scanner.
2. Description of the Related Art
In a well-known optical scanner, a light beam is emitted from a light source device and is deflected and converged as a light spot onto a scanned face by an image forming optical system to perform an optical scanning operation.
For example, a reflective image forming element is proposed as the image forming optical system in the optical scanner in Japanese Patent Application Laying Open (KOKAI) No. 1-220221. This reflective image forming element uses a reflecting mirror having an image forming function in this patent specification.
When the reflective image forming element is used as the image forming optical system, the deflected light beam is reflected on the side of an optical deflector by the reflective image forming element. Therefore, it is necessary to separate the optical path of a light beam incident to the reflective image forming element from the optical path of a light beam reflected on the reflective image forming element. In a simplest method of this optical path separation, directions of the incident and reflected light beams are different from each other in a direction corresponding and parallel to a cross scanning direction.
However, in this method, no scanning line as a locus of the light spot is formed as a straight line so that a so-called scanning line curve is caused as is known.
In the optical scanner, the optical deflector for deflecting the light beam from the light source device is generally constructed by a polygon mirror. In a method for manufacturing the polygon mirror, plastic is molded in a polygonal columnar shape by using a die and a reflecting film is formed as a deflecting reflecting face in a circumferential face portion of this plastic. However, in this manufacturing method, a plastic portion forming the deflecting reflecting face is parallel to a rotating axis of the polygon mirror. Accordingly, it is necessary to open and take out the die in a direction perpendicular to the rotating axis so as to take a plastic molded product out of the die. Therefore, a molding operation is complicated and the molding die is expensive.